The present invention relates to a compound solid fertilizer which makes effective use of artificial or natural inorganic compound mineral matter of little utility value consisting principally of iron, aluminum, magnesium, silica and so forth, or unused organic matter, and a manufacturing method thereof.
Artificial inorganic compound minerals consisting principally of iron, aluminum, calcium, magnesium, potash and so on are, for instance, coal ash discharged from coal thermal power plants and various slag from iron mills, and inorganic compound minerals existing in nature are, for example, serpentine, peridotitite, potash feldspar, magnesium salt, calcium salt, etc. Because of their small contents of useful components, however, these inorganic compound minerals are mostly unused; for example, some of the coal ash and slag are merely utilized as a raw material of cement concrete or artificial aggregate for civil engineering and construction, and they are almost discarded. As a result, great difficulties are now being encountered in finding a dumping ground for such industrial waste. The same is true of various animal and vegetable organics which are discharged from urban agricultural and marine products industries. So, a proposal has recently been made of utilizing such untouched resources as inexpensive complex fertilizers to overcome the abovesaid difficulties in the mining and manufacturing industries, the agricultural and forestry industries and the marine products industry and, at the same time, to prevent effusion of fertilizer and hence reduce farm prices, thereby bringing profits to both the mining and manufacturing industries and the agricultural industry. Moreover, some complex fertilizers based on such a proposal, including those by the present inventor, have already been put to practical use. Since fabrication of such conventional fertilizers involves a firing process which requires much thermal energy it is contrary to what is called a resources-saving principle and inevitably becomes costly and hence is still unsatisfactory.
In view of the above defects, the present inventors have made various studies and, as a result of our studies, have proposed complex fertilizers characterized by the following wet manufacturing method which does not call for such heat treatment as firing (see Japanese Pat. Applns. Nos. 106753/80 and 106754/80). With this method, by adding a phosphoric acid mixture soluton to such inorganic compound minerals as mentioned previously or their mixture with an organic substance, even a substance extremely stable in a natural environment readily decomposes to form a gel-sol mixture of phosphate or silicophosphate. Moreover, supplementary fertilizer components, for instance, chlorides such as potassium chloride and ammonium chloride, sulfates such as ammonium sulfate and potassium sulfate, magnesium, urea and so forth, are mixed with the abovesaid phosphate gel-sol mixture to granulate and harden it through utilization of its self-condensing, hardening and granulating properties, thereby to obtain a complex fertilizer having required fertilizer components contained in the inorganic compounds, the added phosphoric acid and the supplementary fertilizer components. According to this method, the thermal energy necessary for fabrication is sufficient only to promote the granulation and hardening. Since the potassium chloride and ammonium chloirde used as the fertilizer components are inexpensive, low-priced fertilizers can be supplied. Further, the fertilizer components in the inorganic compound minerals and the supplementary fertilizer component such as potassium chloride are included in the gel or sol of phosphate or the like and held in the granules. Besides, hardenability of the granules of the fertilizer fabricated through utilization of the gel-sol mixture is far better than in the case of a fertilizer produced by a conventional compound fertilizer manufacturing method called a mixing method in which prepared sulfate, chloride and phosphate are chemically treated while being mixed together and the mixture is granulated through using an adhesive binder, or by a method called a direct manufacturing method in which sufuric acid and phosphoric acid are combined while being neutralized with urea, ammonia and so forth and the mixture is granulated through using a granulating material such as silicious earth or talc. In addition, the granules produced using the abovesaid gel or sol are less hydroscopic and less soluble than the granules of the fertilizers manufactured by the prior art methods. Accordingly, the fertilizer by the wet manufacturing method is slightly soluble and slow-working as compared with the conventional fertilizers. With the wet manufacturing method, it is therefore possible to offer a fertilizer which prevents the loss of fertilizer components due to effusion and hence is economical and, at the same time, permits lessening the number of fertilizations and the labor therefor and which is less likely to incur water pollution and concentration trouble. The wet manufacturing method is highly advantageous in that it does not necessitate the use of a granulating material for granulation nor does it call for complex equipment such as a multi-stage neutralization reactor and a cooler for the removal of heat of neutralization and energies for operating them unlike the manufacture of the fertilizer consisting principally of phosphoric acid and ammonia according to the aforementioned direct method.
On the other hand, however, the wet manufacturing method employs inexpensive but water-soluble chloride and sulfate from the viewpoint of lowering the cost of fertilizer. Therefore, the fertilizer by this method contains sulfuric acid and chlorine radicals in large quantities as is the case with the fertilizer by the aforesaid mixing method using similar fertilizer components, and it has a serious harmful effect of promoting the deterioration of soil by its acidification resulting from dissolution of the sulfuric acid radical and the chlorine radical. Accordingly, the fertilizer by the wet manufacturing method is inferior in this point to the phosphoric acid-ammonia system fertilizer by the direct manufacturing method and fails to meet the requirements of a good fertilizer.